Biological safety cabinets currently in use have a bottom defining a work surface, a top, three sides, and a transparent front window partially covering the front of the cabinet. The cabinets are used as workbenches for biological cell culturing. Within the cabinet contaminate air is circulated through a filter downwardly through the work area onto the work surface, which work area air, after becoming contaminated, is pulled through a plenum, which surrounds a portion of the cabinet, by a blower and forced downwardly back through the filter again. A high efficiency particulate air ("HEPA") filter is generally employed to filter the air.
The window which covers a portion of the front of the cabinet is transparent to allow for viewing the contents of the work area. There is a 10 inch opening between the lower edge of the window and the threshold of the cabinet to allow for insertion of a user's arms and hands and equipment into the cabinet for placing media into, feeding and examining cell cultures on the work surface and other tasks.
The circulation of filtered air downwardly through the work area as work area air and back through the filter generates an "air curtain" which flows downwardly from the lower edge of the window towards the threshold of the safety cabinet. The threshold has an intake grill which communicates with the plenum. A negative pressure is induced within the plenum which draws the filtered air downwardly through the work area as work area air, along the inside surface of the window and downwardly towards the intake grill creating this "air curtain" effect.
A portion of the ambient air external to the cabinet also enters the intake grill at the threshold of the cabinet along the lower edge of the opening between the threshold and the window. The ambient air which enters the grill, the flow of which is generally parallel to the downwardly directed work area air curtain, is approximately the same velocity as the air curtain as the two enter the intake grill. However, near the top of the opening between the threshold and the window, the ambient air has a velocity much less than the air curtain. The work area air curtain must flow over this plane of ambient air having little velocity, and, due to friction, the velocity of the work area air curtain is reduced. Such a reduction in the velocity of the work area air curtain tends to reduce the effectiveness of the air curtain in preventing an interchange of ambient air with work area air. This phenomenon is compounded at the lateral edges of the air curtain since the velocity of the air curtain must be zero at the cabinet sidewalls, and particularly so at the upper lateral edges of the air curtain in the areas near the upper corners formed by the window and the sides of the cabinet. Therefore, when the air curtain of work area air passes the bottom edge of the window, and especially near the lateral edges of the window, the air curtain is decelerated by the ambient air and the cabinet side walls, causing the air curtain to curl outwardly out of the cabinet. Such is undesirable, as work area air is allowed to escape from the cabinet, and the strength of the air curtain is diminished thereby increasing the tendency of external disturbances, such as persons walking past the cabinet, to induce an interchange of ambient air with work area air.
It has therefore been an objective of the present invention to increase the strength of the air curtain in a biological safety cabinet.
It has been another objective of the present invention to accelerate the velocity of the ambient air external to a biological safety cabinet adjacent its air curtain in order that the difference between the velocities of the air curtain and downwardly traveling ambient air is reduced.
It has been yet another objective of the present invention to reduce the tendency of the air curtain to curl outwardly and escape from a biological safety cabinet.